This project aims to characterize the nature, distribution, and significance of viruses and plasmids carried by spiroplasmas. Accumulated data show that approximately 90% of spiroplasmas, as determined by either electron microscopy or plaque formation on indicator lawns, carry 1 or more viruses: 49% carry only 1, and 40% carry either 2 or 3 simultaneously. About 86% carry also, per strain, 1 to 8 of 38 cryptic plasmids--of which 12 are shared by 1 to 3 other strains. Current studies include examination of new strains for viruses and plasmids, investigation of possible differences in viral susceptibilities among cells in spiroplasma cultures, and attempts to cure plasmid-bearing strains to alter host phenotypes.